All manner of expectations have greeted the changing of the guard on Capitol Hill, almost all of them wholly unrealistic. The arrival of the Democrats, it is said, could help break the logjam in the global trade talks, force the Bush administration to come to the negotiating table on climate change and open the way to a golden era of multilateralism.

Without doubt, this is a pivotal moment. George Bush began his presidency with the determination to use the multilateral system only in so far as it suited what he saw as America's interests. If he could not find agreement - as on Iraq, climate change and trade - the United States was content to go it alone.

Gradually, almost imperceptibly, the mood has changed. It was easy enough to crush Saddam Hussein, but it has been less easy to deal with the political chaos that has followed. It was simple enough to walk away from the table at the trade talks in Geneva in July; much more difficult to orchestrate a series of bilateral deals. Back in 2000, Bush was confident a policy on climate change that said there would be no risks taken with America's economy would resonate with the public; in 2006, after Hurricane Katrina, the president looks out of step with a growing number of his own voters, many of whom accept the arguments in favour of curbs on greenhouse gas emissions.

So the Democrats take control of the US legislature as the world has to decide whether it wants to act together to solve the problems that confront it. And, yes, there are reasons to expect a shift in emphasis in Washington, but that is all it is going to be. These New Democrats move slowly and only when the focus groups tell them the public is receptive. The message from the midterm elections was not that the American public was demanding greater engagement with the wider world; it was that they wanted something done about Iraq.

The United Nations human development report released last week is a small but important test of whether there is a hope of greater international cooperation, since it documents with great clarity how children in sub-Saharan Africa are dying every day as a result of failures of sanitation that were remedied in Britain more than 150 years ago.

About 1.8m children die of diarrhoea every year - almost 5,000 each day - and those living in the slums of Nairobi and Jakarta are paying more for their water than people in London or New York. It is not the know-how that is lacking to save avoidable deaths but rather the resources and if the international community can't deliver on something as basic as clean water, there's not much chance of it delivering on anything else.

There are those who detect a shift in the tectonic plates. Gordon Brown hailed the launch last week of his long-awaited International Finance Facility for Immunisation (IFFIM) as a sign that the international community was starting to deliver on the promises made during Britain's presidency of the G8 in 2005. The chancellor's view is that the public pressure surrounding the Gleneagles summit last year has continued to push politicians to act and that debt relief and commitments to provide more aid are a sign of this.

Brown, like Tony Blair, believes that all is not lost with the Doha Round of trade talks. While Washington has been beavering away trying to sign bilateral deals, Britain has kept faith with the multilateral approach, and it expects the talks - in deep freeze since July - can be reinvigorated now that the US midterm elections are over.

Swansong

Nor is this an end to the upbeat mood in Whitehall. Blair, armed with the Stern report on the economics of climate change, is trying to agree the framework for a post-Kyoto agreement on cutting emissions that would embrace the countries that really matter - the US, China and India. It is, the prime minister, argues, possible for this to be ready by the time of the German G8 summit next year, which - conveniently enough - looks like being his international swansong as well.

On past form, this looks like being a forlorn hope. That's not to say that a deal by next summer would be unwelcome; on the contrary, the message from the Stern review is that the sooner action is taken the better. Indeed, the report rather pulls its punches; it establishes a strong case for capping emissions of greenhouse gases in the atmosphere at 450 parts per million, but then argues that this is a rather unrealistic target and so settles for 550ppm.

Sir Nicholas Stern says capping emissions at this level would risk serious damage from climate change, a view supported by a paper from the Institute for Public Policy Research last week. The authors, Paul Baer and Michael Mastrandrea, say that stabilising greenhouse gas emissions at between 450ppm and 550ppm (in the real world that would mean countries taking the soft option and plumping for 550ppm), as Stern suggests, would carry a medium to high risk of temperatures rising by more than 2C above pre-industrial levels. Action, they say, is needed and it is needed now. "Given that global emissions trajectories are currently heading in the opposite direction, the level of effort required to bend the global emissions curve in time is Herculean."

For those who accept that keeping global temperature rises below 2C is crucial (and not everybody accepts that stricture) they conclude that a "crash programme" is needed to reduce emissions to ensure the peak is reached between 2010 and 2013. On this basis, Blair is right to be pushing the Germans hard about next year's G8 agenda and presumably hopes that the wriggle room offered to the Americans by a 550ppm target will bring Bush, prodded by Congress, to the negotiating table.

A more likely outcome, however, is that the trade talks remain in cold storage and negotiations on climate change move forward at glacial pace. Why? Because when it comes to economic policy, the Democrats' approach has owed more to protectionism than internationalism, calling for policies that would safeguard American jobs from the forces of globalisation.

Pascal Lamy, the director-general of the World Trade Organisation, and from a left-of-centre political background himself, was recently in Washington trying to ginger up the Democrats to push for a deal on trade once the midterm elections were over. A Doha deal, Lamy said, would do no damage to blue-collar Democrats working in manufacturing but instead target the subsidies being trousered by rich (ie Republican) farmers. There is something in this argument, since the change to the world trading system that has had the biggest impact on American jobs over the past decade was the admission of China to the WTO in 2001. What's more, slapping duties on Chinese imports in retaliation for the undervaluation of the yuan, as proposed by the Schumer-Graham bill, would be something of a double-edged sword, since the move would add to inflation at a time when the Federal Reserve is already concerned about the cost of living in America.

The upshot of the Schumer-Graham bill - if it is passed by Congress - would almost certainly be higher interest rates in the US and it's not easy to see how that would be good for jobs.

But for the time being, the Democrat message is more likely to be "troops out" and "import tariffs" than Doha and climate change. And that, let's be in no doubt about it, could have very severe consequences.